Apparatus for generating an image from a digital video signal

ABSTRACT

An apparatus for generating a high quality image from a digital video signal includes a system for gamma correcting the digital video signal with a digital look up table and for converting the resultant digital signal to an analog video signal. Another circuit generates a triangular wave reference pattern signal and a comparator compares the analog video signal with the triangular wave reference pattern signal to form a pulse-width-modulated signal. A raster scanning print engine producing, for example, a laser beam, scans over a recording medium in accordance with the pulse-width-modulated signal, thereby forming an image of high quality on the recording medium of a print engine. This apparatus can also be used with an analog video signal by first converting the analog video signal to a digital video signal with an analog to digital converter.

This application is a reissue of U.S. Pat. No. 4,800,442, which issuedon application Ser. No. 51,154, filed May 18, 1987, which is acontinuation of application Ser. No. 931,941 filed 11/19/86, nowabandoned, which was a continuation of application Ser. No. 765,938filed 8/15/85, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus for generating an imagefrom a digital video input signal. The apparatus is improved so as toreproduce an image with high quality.

2. Description of the Prior Art

In the past, methods generally referred to as the dither method and thedensity pattern method have been proposed for reproducing images of halftones. These known methods, however, cannot provide satisfactorygradation of dot size when the size of the threshold dot matrix is smalland, therefore, require the use of a threshold matrix having a largersize. This is turn reduces the resolution and undesirably allows thetexture of the image to appear too distinctive due to the periodicstructure of the matrix. Therefore, deterioration of the quality of theoutput image results.

In order to mitigate the above described problems, it has been proposedto modify the dither method so as to allow finer control of the dot sizeby the use of a plurality of dither matrices. This method, however,requires a complicated circuit arrangement for obtaining synchronism ofoperation between the dither matrices so that the system as a whole islarge in size, complicated in construction, and slow. Thus, there is apractical limit in the incremental increase of dot size and theresultant increment of density available by the use of a plurality ofdither matrices. In U.S. Pat. No. 3,916,096, a method of improving theconventional screening process is described. As set forth in this U.S.Pat. No. 3,916,096, at column 8, lines 19 through 31:

-   -   The conventional screening process when applied to a scanned        image can be regarded as a form of pulse-width-modulation        whereby a line of length X is laid down and repeated at        intervals of Y. The percentage transmission (or reflection) of        the reproduced image is then Y−X/Y [sic. should read (Y−X)/Y].        To be a linear process (Y−X) must be directly proportional to        the amplitude of the scanned video signal where the signal        amplitude represents the percentage optical transmission of the        recorded original image. A way of achieving this is by comparing        the amplitude of the video signal with a sawtooth wave form and        laying a line forming a portion of a dot whenever the sawtooth        is larger than the video signal.        See also U.S. Pat. No. 4,040,094, which relates to similar        subject matter.

However, even if the method described in this patent is used in anapparatus for reproduction of an image, the precision of gradationreproduction deteriorates due to the delay of response of the apparatus.

The conventional method described in U.S. Pat. No. 3,916,096, produces alinear mapping from the analog video signal to the pulse-width-modulatedsignal. As is known in the art of printing, this linear mapping does notproduce acceptable results because of the non-linear distortionsintroduced in the half-tone printing process, in particular when usedwith a laser beam print engine. Therefore, to obtain high qualityhalf-tone printing, a method of non-linear mapping must be found. And,the method disclosed in the noted U.S. Patent, as quoted above, uses acomplex arrangement to allow the use of different sawtooth waveforms onsuccessive scans.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide an imageprocessing apparatus, for generating an image from a digital videosignal, that can overcome the problems of the prior art described above.

Another object of the present invention is to provide an imageprocessing apparatus, for generating an image from a digital videosignal, that permits reproduction of images with high quality.

Still another object of the present invention is to provide an imageprocessing apparatus, for generating an image from a digital videosignal, that can provide, with a very simple arrangement, a superiorquality half-tone image.

Another object of the present invention is to provide an imageprocessing apparatus, for generating an image from a digital videosignal, that permits reproduction of images with high quality at highspeed.

A further object of the present invention is to provide an imageprocessing apparatus, for generating an image from a digital videosignal, that can reproduce tone information with a high gradation andwithout impairing resolution.

Still another object of the present invention is to provide an imageprocessing apparatus that can correct the tonal properties of the videoimage by providing a non-linear mapping of the video signal onto apulse-width-modulated signal with a very flexible arrangement.

In accordance with a preferred embodiment, the image processingapparatus of the present invention processes a digital image inputsignal and includes a raster scanning print engine for generating aseries of successive scanning lines. A pulse-width-modulated signalgenerator generates a pulse-width-modulated signal from a digital imageinput signal input to the apparatus. A circuit then applies thepulse-width-modulated signal to the print engine to cause it to generateeach line as a succession of line segments. The lengths of the linesegments are controlled to produce a variable density line screen fromthe line segments with the line screen comprising a plurality of columnsof the line segments.

In accordance with another aspect of a preferred embodiment of thepresent invention, the image processing apparatus includes a patternsignal generator for generating a pattern signal of predeterminedperiod. A pulse-width-modulated signal generator then generates apulse-width-modulated signal in accordance with the video signal and thepattern signal that can be utilized by a raster scanning print engine orimage forming device to form an image.

More specifically, the print engine scans lines on a recording mediumwith a beam in accordance with the pulse-width-modulated signal, and asynchronizing signal generator generates a synchronizing signal for eachline scanned on the recording medium. The pattern signal generatorgenerates the pattern signal of predetermined period in accordance withthe synchronizing signal.

In accordance with still another aspect of the invention, the digitalinput signal has a characteristic, and a characteristic convertingdevice converts the characteristic in order to produce a converteddigital video signal. This signal is converted to an analog video signalby a digital to analog converter. A pulse-width-modulated signal isthereafter generated from this analog video signal and the patternsignal.

Other aspects, features, and advantages of the present invention willbecome apparent from the following detailed description of the preferredembodiments taken in conjunction with the accompanying drawing, as wellas from the concluding claims.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a simplified schematic illustration of a preferred embodimentof the apparatus for generating an image from a digital video signal inaccordance with the present invention;

FIG. 2 shows waveforms of signals obtained at different portions of theapparatus for generating an image from a digital video signal shown inFIG. 1.

FIG. 3 shows how FIGS. 3A and 3B are assembled together to illustratedetails of the embodiment of the apparatus for generating an image froma digital video signal shown in FIG. 1;

FIG. 4 is a schematic illustration of an optical scanning system in alaser beam printer to which the invention is applicable;

FIG. 5 shows waveforms of signals obtained at different portions of thecircuit shown in FIGS. 3A and 3B;

FIG. 6 is an illustration of triangular wave signals formed in thecircuit shown in FIGS. 3A and 3B;

FIGS. 7(a) to 7(c) are illustrations of how triangular wave signals maybe adjusted in the embodiment of the invention;

FIG. 8 is an illustration of a look-up table of a gamma converting ROM12;

FIG. 9 is a diagram showing the relationship between input video signalsand converted video signals;

FIGS. 10(a) and 10(b) illustrate the relationship between the scanninglines and the conversion table as used;

FIG. 11 is a circuit diagram of a circuit for causing phase shift oftriangular wave signals between lines;

FIG. 12 is an illustration of triangular wave signals appearing inrespective lines at different phases; and

FIG. 13 is an illustration of another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the invention will be described in detailherein with reference to the accompanying drawing.

Referring first to FIG. 1 schematically showing an embodiment of theinvention, a digital data output device 1 is adapted to receive ananalog video data from a CCD sensor or a video camera (neither of whichis shown) and to perform an A/D (analog-to-digital) conversion of theanalog video signal so as to convert that signal into a digital videosignal, where each picture element (pixel) is represented by apredetermined number of bits carrying tone information. The digitalvideo signal may be temporarily stored in a memory or, alternatively,may be supplied from an external device by, for example,telecommunication. The signal from the digital data output device 1 isused as the address for a digital look-up table for gamma correction 9.The resultant output, which in the preferred embodiment is an eight (8)bit digital number ranging from OOH to FFH representing 256 possibletonal gradation levels as described further below, is converted backinto an analog signal by means of a D/A (digital to analog) converter 2so as to form an analog signal which is updated for each pictureelement. The analog video signal representing the picture elements isfed to one of the input terminals of a comparator circuit 4.Simultaneously, analog reference pattern signals having a triangularwaveform are produced by a pattern signal generator 3 at a periodcorresponding to the desired pitch of the half-tone screen. The patternsignals (a triangle wave) are fed to the other input terminal of thecomparator circuit 4. Meanwhile a horizontal synchronizing signalgenerating circuit 5 generates horizontal synchronizing signals forrespective lines, while an oscillator (reference clock generatingcircuit) 6 generates reference clocks. In synchronism with thehorizontal synchronizing signal, a timing signal generating circuit 7counts down the reference clocks, to, for example, ¼ period. The signalderived from the timing signal generating circuit 7 is used as the clockfor the transfer of the digital video signal and also as the latchtiming signal for the D/A converter 2.

In the embodiment described, since the apparatus is intended for use ina laser beam printer, the horizontal synchronizing signal corresponds toa beam detect (BD) signal which is known per se. The comparator circuit4 compares the level of the analog video signal with the level of thepattern signal of triangular waveform and produces apulse-width-modulated signal. The pulse-width-modulated signal issupplied to, for example, the laser modulator circuit of a rasterscanning print engine 8 for modulating the laser beam. As a result, thelaser beam is turned on and off in accordance with the pulse widththereby forming a half-tone image on the recording medium of the rasterscanning print engine 8.

FIG. 2 shows the waveforms of signals obtained in certain components ofthe apparatus shown in FIG. 1. More specifically, the portion (a) ofFIG. 2 diagrammatically shows the reference clocks generated by theoscillator 6, while the portion (b) shows the horizontal synchronizingsignal mentioned above. The portion (c) shows the pixel clocks which areproduced by counting down the reference clocks with the timing signalgenerating circuit 7. More specifically, the pixel clock shown in theportion (c) of FIG. 2 is the signal which is obtained by counting downthe reference clocks into ¼ period by the operation of the timing signalgenerating circuit 7 in synchronism with the horizontal synchronizingsignal. The pixel clock thus obtained is delivered to the D/A converter2 to be used as the digital video signal transfer clock. The portion (d)of FIG. 2 shows the pattern signal synchronizing clock (screen clock)which is obtained by counting down the reference clock into 1/12 periodby operation of the timing signal generating circuit 7 in synchronismwith the horizontal synchronizing signal. In the illustrated case, onepattern signal synchronizing clock is generated for every three pixelclocks. The pattern signal synchronizing or screen clock thus obtainedis delivered to the pattern signal generator 3 to be used as thesynchronizing signal in the generation of the pattern signal. Theportion (e) of FIG. 2 shows the digital video signal which is outputfrom the digital data output device 1. And the portion (f) shows theanalog video signal after the D/A conversion conducted by the D/Aconverter 2. It will be seen from the portions of FIG. 2 that pictureelement data of analog level are produced in synchronism with the pixelclocks. It will also be seen that the density of image becomes higher,i.e., approaches black, as the level of the analog video signal rises.

As shown by a solid line curve in the portion (g) of FIG. 2, the outputfrom the pattern generator 3 is obtained in synchronism with the clocksshown in the portion (d) and is input to the comparator circuit 4. Thebroken line curve in the portion (g) of FIG. 2 shows the analog videosignal shown in the portion (f). This video signal is compared by thecomparator circuit 4 with the pattern signal of triangular waveformderived from the pattern signal generator 3 so that the analog videosignal is converted into a pulse-width-modulated signal as shown in theportion (h) of FIG. 2.

The described embodiment of the invention permits a substantiallycontinuous or linear pulse modulation and, hence, ensures a highgradation of the image output by virtue of the fact that the digitalvideo signal is converted into an analog video signal which is thencompared with the triangular wave signal of a predetermined period.

It is to be noted also that, in the described embodiment of theinvention, the pattern signal synchronizing clock (screen clock) forgeneration of the pattern signal, e.g., the triangular wave signal, isgenerated in synchronism with the horizontal synchronizing signal bymaking use of reference clocks having a frequency much higher than thatof the pattern signal synchronizing signal. Therefore, the jitter of thepattern signal derived from the pattern signal generator 3, e.g., theoffset of the pattern signal from one scan line to the next, is reducedto less than 1/12 of the period of the pattern signal. This precision isrequired to insure a high quality half-tone reproduction in which theline screen is formed uniformly and smoothly from one scan line to thenext. Therefore, the density information can be accurately pulse-widthmodulated by making use of this pattern signal which has a smallfluctuation, so that the image can be reproduced with high quality.

FIG. 4 is a schematic perspective view of the optical scanning systemincorporated in the laser beam printer (a raster scanning print engine)to which the present invention is applied. The scanning system has asemiconductor laser adapted to emit a laser beam modulated in accordancewith the pulse-width-modulated signal mentioned above.

The optical laser beam modulated by the semiconductor laser 21 iscollimated by a collimator lens 20 and is optically deflected by apolygonal mirror (applying means) 22 having a plurality of reflectingsurfaces. The deflected beam is focused to form an image on aphotosensitive drum 12 by an image forming lens 23 referred to as an fθlens, so as to scan the drum 12. During the scanning by the beam, andwhen the beam reaches the end of each scanning line, it is reflected bya mirror 24 and is directed to a beam detector 25. The beam detection(BD) signal produced by the beam detector 25 is used as the horizontalsynchronizing signal as is known. Thus, in the described embodiment, thehorizontal synchronizing signal is constituted by the BD signal.

It will be seen that the BD signal is detected for each of the lines ofscanning by the laser beam and is used as the timing signal for thetransmission of the pulse-width-modulated signal to the semiconductorlaser.

As used in the subject specification in description of the preferredembodiments and as used in the concluding claims, the term“line-segment” means a dot which is formed on a recording medium, thelength (size) of which is variable in accordance with the width of thepulse width in the supplied pulse-width-modulated signal.

The apparatus for generating an image from a digital video signal of theinvention will be described more fully with specific reference to FIGS.3A and 3B which show details of the apparatus shown in FIG. 1.

As stated before, the preferred embodiment described herein makes use ofthe BD signal as the horizontal synchronizing signal. The BD signal,however, is basically asynchronous with the pixel clock and, therefore,would normally cause jitter in the horizontal direction. In thedescribed embodiment, therefore, jitter is reduced to less than ¼ of thewidth of a pixel, by making use of an oscillator 100 that can producereference clocks (72M-CLK) (72 megahertz clock) of a frequency which is4 times higher than that of the pixel clocks. A BD synchronizing circuit200 is used for this purpose. The reference clock (72M-CLK) from theoscillator 100 is supplied to D latches 201, 202, and 203 through abuffer 101, while the BD signal is input to the data terminal D of the Dlatch 201 through a terminal 200a so as to be synchronized with thereference clocks. In addition, the BD signal is delayed by the D latches202 and 203 by an amount corresponding to 2 (two) reference clockpulses. The BD signal thus delayed is delivered to one of the inputterminals of a NOR gate 103, while the other input terminal of the NORgate 103 receives the inverted output of the D latch 201. The outputfrom the NOR gate 103 is input to one of the input terminals of a NORgate 104, while the other input terminal of the NOR gate 104 receivesthe output of a flip-flop circuit 102.

With this arrangement, the flip-flop circuit 102 produces clocks(36M-CLK) (36 megahertz clock) which are obtained by dividing thefrequency of the reference clock by 2 (two). Thus, the output (36M-CLK)from the flip-flop circuit 102 is synchronous with the BD signal towithin one period of the clock 72M-CLK.

The output of the D latch 203 is delayed by the D latches 204, 205, and206 by an amount corresponding to 3 (three) pulses of the output(36M-CLK) of the flip-flop circuit 102.

The inverted output from the D latch 201 and the output from the D latch206 are delivered to a NOR gate 207, so that an internal horizontalsynchronizing signal (BD-Pulse) is formed in synchronism (within oneperiod) with the reference clock.

FIG. 5 shows the timing of the signals obtained at various portions ofthe BD synchronizing circuit 200. More specifically, A-1 shows the BDsignal, A-2 shows the reference clock (72M-CLK) produced by theoscillator 100, and A-3 shows the inverted output from the D latch 201,obtained by synchronizing the BD signal with the reference clock(72M-CLK). A-4 shows the output from the D latch 203, obtained bydelaying the signal A-3 by an amount corresponding to 2 (two) referenceclock pulses. A-5 shows the clock (36M-CLK) output from the flip-flopcircuit 102, A-6 shows the output from the D latch 206, obtained bydelaying the signal A-4 by an amount corresponding to 3 (three) pulsesof the clocks (36M-CLK), and A-7 shows the internal horizontalsynchronizing signal (BD-Pulse). It will be seen that the internalhorizontal synchronizing signal (BD-Pulse) rises in synchronism with therise of the first reference clock (72M-CLK) after the rise of the BDsignal, and is held at level “1” for a period corresponding to 8 (eight)pulses of the reference clock. This internal horizontal synchronizingsignal (BD-Pulse) constitutes the reference for the horizontal drivingof the circuit of this embodiment.

An explanation of the video signals will now be made again withreference to FIGS. 3A and 3B. The pixel clocks (PIXEL-CLK) are formed bydividing the frequency of the signal (36M-CLK) by 2 (two) by means ofthe J-K flip-flop circuit 105. A 6-bit digital video signal is latchedin the D latch 10 by the pixel clock (PIXEL-CLK), and the output isdelivered to a ROM 12 for gamma conversion. The 8-bit video signalproduced through the conversion by the ROM 12 is further converted intoan analog signal by the D/A converter 13 and is delivered to one of theinput terminals of the comparator 15 in order to be compared with thetriangular wave signal explained below. The pulse-width-modulated signalobtained as a result of the comparison is delivered to the laser driverof a raster scanning print engine.

Still referring to FIGS. 3A and 3B, reference numeral 300 designates ascreen clock generating circuit which generates the screen clock, i.e.,the analog reference pattern signal synchronizing clock, which is usedas the reference for the generation of the triangular wave signal. Acounter 301 is used as a frequency divider for dividing the frequency ofthe signal (36M-CLK) output from the flip-flop circuit 102. The counter301 has input terminals D, C, B, and A which are preset withpredetermined data by means of a switch 303. The ratio of the frequencydivision is determined by the values set at these input terminals D, C,B, and A. For instance, when the values “1”, “1”, “0”, and “1” are setin the terminals D, C, B, and A, respectively, the frequency of thesignal (36M-CLK) is divided into ⅓.

Meanwhile, horizontal synchronization is attained by the NOR gate 302and the (BD-Pulse) signal. The frequency of the divided signal isfurther divided into ½ by a J-K flip-flop circuit 304, so that a screenclock having a duty ratio of 50% is formed. A triangular wave generatingcircuit 500 generates triangular waves by using this screen clock as thereference.

FIG. 6 shows waveforms of signals appearing at various components of thescreen clock generating circuit 300. (It is noted, however, that thescales of FIGS. 5 and 6 are different). More specifIcally, B-1 shows theinternal synchronizing signal (BD-PULSE), B-2 shows the signal (36M-CLK)and B-3 shows the screen clock (SCREEN CLK) as obtained when values “1”,“1”, “1”, “0” are set in the terminals D, C, B, A of the counter 301,respectively. B-4 represents the triangular wave signal as obtained whenthe screen clock B-3 is used as the reference. On the other hand, B-5shows the screen clock (SCREEN CLK) as obtained when values “1”, “1”,“0”, “1” are set in the input terminals D, C, B, A of the counter 301.B-6 shows the triangular wave signal as obtained when the screen clock(SCREEN CLK) shown in B-5 is used as the reference obtained. It will beseen that the period of the triangular wave signal shown by B-4corresponds to 2 (two) picture elements, while the period of thetriangular wave signal shown by B-6 corresponds to 4 (four) pictureelements. Thus, the period of the triangular wave signal can be variedas desired by appropriately setting the switch 303. In the embodimentdescribed, the period of the triangular wave is changeable between aduration corresponding to 1 (one) picture element and a durationcorresponding to 16 (sixteen) picture elements.

The triangular wave signal generating circuit 500 will now be described,again with reference to FIGS. 3A and B. The screen clock (SCREEN CLK) isreceived by the buffer 501, and the triangular wave is generated by anintegrator comprising by a variable resistor 502 and a capacitor 503.The triangular wave signal is then delivered to one of the inputterminals of the comparator 15 through a capacitor 504, a protectiveresistor 506, and a buffer amplifier 507. The triangular wave signalgenerating circuit 500 has two variable resistors, namely, variableresistor 502 for adjusting the amplitude of the triangular wave signal,and a variable resistor 505 for adjusting the bias or offset of thetriangular wave signal. The adjustment of the amplitude and the offsetof the triangular wave signal by the variable resistors 502 and 505 isconducted in a manner which will be explained with reference to FIGS.7(a) to 7(c). In FIG. 7(a), a triangular wave signal Tri-1 beforeadjustment is shown by a solid line curve. By adjusting the variableresistor 502, the signal Tri-1 is changed into an amplified triangularwave signal Tri-2 shown by a broken line curve. Then, the variableresistor 505 can be adjusted to shift or adjust the offset of the waveso as to form a triangular wave signal Tri-3 shown by aone-dot-and-one-dash line curve. It is thus possible to obtain atriangular wave signal having the desired amplitude and offset.

As stated before, the triangular wave signal thus formed is compared bythe comparator 15 with the output of the D/A converter 13, i.e., withthe analog video signal. The relationship between the triangular wavesignal and the analog video signal is preferably such that the maximumlevel of the triangular wave equals the level of the output of the D/Aconverter 13 as obtained when the input to the converter 13 has themaximum level (FFH, where H indicates a hexadecimal number), while theminimum value of the triangular wave signal equals the level of theoutput of the D/A converter 13 as obtained when the input to thisconverter has the minimum level (OOH). Since the amplitude and theoffset of the triangular wave can be controlled as desired, it ispossible to obtain this preferred condition without difficulty.

More particularly, according to the invention, the amplitude and theoffset of the triangular wave signal are adjusted in the followingmanner. In general, a laser driver for emitting a laser beam has acertain delay time in its operation. The delay time until the laser beamis actually emitted is further increased due to the beam emittingcharacteristics of the laser. Therefore, the laser does not startemitting the laser beam until the width of the pulse input to the driverexceeds a predetermined value. This means that, in the case where theinput signal is a series of periodic pulses as in the case of thedescribed embodiment, the laser does not emit a beam unless the inputsignal pulse has a duty ratio greater than a predetermined value.Conversely, when the duty ratio of the pulse is increased beyond acertain level, i.e., when the period of low level of the pulse isshortened, the laser tends to stay on, that is, the beam is continuouslyemitted. For these reasons, if the adjustment of the triangular wavesignal is conducted in the manner shown in FIG. 7(b), the gradationlevels around the minimum level (OOH) and near the maximum level (FFH)are omitted from the 256 gradation levels of the input data which may beinput to the D/A converter 13, so that the gradation deterioratesundesirably. In the embodiment described, therefore, the variableresistors 502 and 505 are adjusted so that the pulse width just belowthat which will cause the laser to begin emission is obtained at the OOHlevel of the data input to the D/A converter 13, and so that the pulsewidth which will render the laser continuously on is obtained at the FFHlevel of the data input to the D/A converter 13. This manner ofadjustment of the variable resistors 502 and 505 is shown in FIG. 7(c).

As will be understood from FIG. 7(c), this preferred embodiment isdesigned so that the comparator 15 produces an output pulse of a certainpulse width (a pulse width just below that which will cause the laser tobegin emission) when the minimum input data OOH is supplied to the D/Aconverter 13. The preferred embodiment is also designed so that, whenthe maximum input data FFH is supplied to the D/A converter 13, thecomparator produces output pulses the duty ratio of which is not 100%but which is large enough to allow the laser to emit the beamcontinuously. This arrangement permits the emission time of the laser tovary nearly over the entire range of the 256 gradation levels of theinput data, thus ensuring high gradation of the reproduced image.

It should be understood that the method described above is not limitedto a laser printer but may also be utilized in an ink jet printer, athermal printer or other raster scanning devices.

The ROM 12 for gamma conversion will now be explained in detail withreference to FIG. 8. The ROM 12 is provided to allow a high gradation ofdensity in the reproduced image. Although the described embodimentemploys a ROM having a capacity of 256 bytes as ROM 12, a capacity of 64bytes is basically enough because the input digital video signal is a6-bit signal. FIG. 8 shows the memory map of the ROM 12 for gammaconversion. Since this ROM has a capacity of 256 bytes, it can contain 4(four) separate correction tables, namely TABLE-1 including addressesOOH to 3FH, TABLE-2 including addresses 4OH to 7FH, TABLE-3 includingaddresses 8OH to BFH, and TABLE-4 including the addresses COH to FFH.

FIG. 9 shows a practical example of the input-output characteristics ofeach of the conversion tables, i.e., the relationship between the inputvideo signals and the converted output video signal. As will be seenfrom this Figure, the 64 (sixty-four) levels of the input video signalare converted into levels 0 to 255 (OOH to FFH) in accordance with therespective conversion tables. The change-over between the conversiontables can be made by varying the signal applied to upper terminals A6and A7 of the ROM 12 as shown in FIGS. 3A and 3B. The describedembodiment is designed to allow this switching for each line, by theoperation of a circuit 400 shown in FIG. 3A. In operation, the internalhorizontal synchronizing signal (BD-Pulse) is input to a counter 401 theoutput of which is delivered through terminals QA and QB to theterminals A6 and A7 of the ROM 12. The counter 401, in cooperation withan RCO inverter 402 and a switch 403 constitutes a ring counter, so thatthe period of switching of the conversion table can be varied inaccordance with the state of the switch 403. For instance, when theswitch 403 has the state “1” (at terminal B), “1” (at terminal A),TABLE-4 is always selected, whereas, when the state of the switch 403 is“1” (at terminal B), “0” (at terminal A), TABLE-4 and TABLE-3 areselected alternately. When the switch 403 has the state of “0” (atterminal B), “0” (at terminal A), TABLE-1, TABLE-2, TABLE-3, and TABLE-4are successively selected for successive lines, as shown in FIG. 10a.Moreover, it is possible to improve the gradation by changing theconversion table for successive lines.

In general, in the electrophotographic reproduction of an image, thegradation is more difficult to obtain in the light portion of the imagethan in the dark portion of the image. Therefore, as in the exampleshown in FIG. 9, the conversion tables are substantially duplicated inthe dark portions of the image and differ in the light portion so as toprovide optimal gradation.

In the preferred embodiment, the switching of the table can also be madein the direction of the main scan by the laser beam.

More specifically as shown in FIGS. 3A and 3B, a signal can be formed bydividing the frequency of the screen clock (SCREEN-CLK) by 2 (two) bymeans of a J-K flip-flop circuit 404, inputting the resulting signal toone input terminal of an exclusive OR circuit 406, the other input ofwhich is connected to terminal QB of the counter 401 and the outputterminal of which is then connected to ROM 12 through a latch 11. Withthis arrangement, it is possible to change the conversion table in astaggered manner as shown in FIG. 10(b), thus attaining a furtherimprovement in the gradation. A reference numeral 405 denotes a switchfor selecting either switching of the table in the staggered mannerdescribed above or not so switching. The staggered switching of thetable is selected when this switch has the “1” level and is not selectedwhen the switch has the “0” level. The numerals appearing in frames ofthe table shown in FIG. 10(b) represent the numbers of the selectedconversion tables 1 to 4. Thus, the period of the screen clock in theembodiment corresponds to the period of 3 (three) pixel clocks.

It will be understood from the description provided above that thescanning lines produced by the laser in accordance with data from theconversion tables of the ROM 12 are each generated as a succession ofline segments. The line segments of successive scanning linescollectively form a plurality of columns that define a line screen.

More particularly, when the video signal processed by the circuit shownin FIGS. 3A and 3B is directly delivered to a reproducing means such asa laser beam printer, the reproduced image has a structure with verticalcolumns (in the described embodiment, the line screen is composed ofvertical columns of line segments of successive scanning lines whichform in the reproduced image) due to the fact that the phase of thetriangular wave signal is the same as that of the internal horizontalsynchronizing signal (BD-Pulse) for each line. The circuit in thepresent embodiment is one in which the triangular wave is formed afterthe reference clocks are counted by 12 (twelve) from the rise of theBD-Pulse signal. The timing for the generation of triangular waves isthe same for each line, and so each phase of the triangular waves oneach line is the same. The image data is output from the digital dataoutput device 1 as stated above. The digital data output device 1outputs image data with a predetermined timing in synchronism with asignal equivalent to the BD-Pulse signal. More particularly, the dataoutput device 1 is adapted to receive the BD signal. This device 1starts to count the reference clock after receiving the BD signal, andbegins transmission of the image data after counting the referenceclocks up to a predetermined number. As a consequence, the timing oftransfer of the image data necessary for image reproduction is the sameon each line, and a high quality reproduced image with no image jittercan be produced. As the timing of the generation of the triangular wavesand the timing of transfer of the image data necessary for imagereproduction have the same relation on all of the lines, the reproducedimage has its vertical column structure with no image jitter, which iseffective, for example, in reducing a particular Moire pattern. Againthis vertical column tructure comprises a line screen having a verticalcolumnar axis extending at an angle, that is perpendicular to the rasterscanning lines.

It is also possible to obtain a reproduced image having a structurecomprising oblique line screen columns, if the phase of the triangularwave signals is made to be offset slightly for successive lines. This iseffective in reducing the Moire pattern which appears undesirably whenan original dot image is read and processed. The angle of inclination ofthe oblique columns can be determined as desired by suitably selectingthe amount of shift of the phase of the screen clocks for successivelines. For instance, a reproduced image comprising scanning lines havingoblique columns inclined at 45 degrees can be obtained by shifting thetriangular wave signal by an amount corresponding to one pictureelement, i.e., by phase shifting the triangular wave signal 120 degreesfor each of the successive columns. FIG. 11 shows a circuit forreproducing an image comprising oblique columns. More specifically, areproduced image comprising oblique columns can be obtained by usingthis circuit in place of the screen clock generating circuit 300 in thecircuit shown in FIG. 3.

Referring again to FIG. 11, the internal horizontal synchronizing signal(BD-Pulse) is latched by the pixel clocks (PIXEL-CLK) by means of Dlatches 356 and 357, so that three internal horizontal synchronizingsignals (BD-Pulse) having different phases are produced. Then, one ofthese three internal horizontal synchronizing signals (BD-Pulse) isselected for each line by operation of a counter 358, inverters 359 and360, and gate circuits 361 to 367. The selected signal is input as aLOAD signal to a counter 351, thereby changing the phase of the screenclocks for successive lines. The counter 351 is adapted to divide thefrequency of the signal (36M-CLK) into ⅓, while the J-K flip-flopcircuit 354 further divides the frequency of the output from the counter351 into ½. With this arrangement, it is possible to generate one screenclock for every three picture elements.

FIG. 12 shows timing of the screen clock generated by the circuit ofFIG. 11 and the triangular wave signal for successive lines. These threetriangular wave signals are generated in sequence of each set of each 3lines.

When the reference pattern signal is generated in synchronism with agroup of picture elements as in the case of the embodiment described, itis possible to shift the synchronizing signal used in the generation ofthe pattern signal by an amount corresponding to one half of thereference pattern signal period for each successive set of scan linesequal to the width of the pattern signal. Such a method allows theposition of the center of growth of the pulse width to be shifted ineach of successive lines, so that the output image can have anappearance resembling that produced by half-tone dots arranged alongoblique lines.

In the circuit shown in FIGS. 3A and 3B, the ROM 12 is used for thepurpose of gamma conversion. This, however, is not the only elementsuitable for this purpose and the ROM 12 may be replaced by an S-RAMconnected to the DATA BUS line of a computer. With such an arrangement,it is possible to rewrite the gamma conversion table as desired inaccordance with, for example, a change in the kind of the original, thusincreasing the adaptability of the apparatus of the invention.

FIG. 13 shows an example of a circuit which is usable in place of theROM 12 in the circuit shown in FIGS. 3A and 3B. This circuit has, aswill be seen from this Figure, an S-RAM l2a for gamma conversion, adecoder 30, a microcomputer 31 for rewriting the gamma conversiontables, tri-state buffers 32 and 33, and a bi-directional tri-statebuffer 34.

The mode changing switches 304, 403 and 405 in the circuit shown inFIGS. 3A and 3B may be controlled by the microcomputer 31 so as toincrease the flexibility of the system as a whole.

Although the invention has been described with reference to specificembodiments and in specific terms it is to be understood that thisdescription is only for illustrative purposes and that various otherchanges and modifications are possible without departing from the scopeof the invention.

1. An image processing apparatus responsive to a digital input signal,said apparatus comprising: a raster scanning print engineimage formingmeans for generating a series of successive scan lines; means forgenerating an analog pattern signal; means for generating apulse-width-modulated signal from a digital input signal input to saidapparatus and from said analog pattern signal; and means for applyingsaid pulse-width-modulated signal to said print engine image formingmeans to cause said print engine image forming means to generate eachsaid scan line as a succession of line-segments, the lengths of whichare controlled in accordance with said pulse-width-modulated signal toproduce a variable density line screen from said line segments, saidline screen comprising a plurality of columns of said line segments. 2.An image processing apparatus responsive to a digital input signal, saidapparatus comprising: a raster scanning print engine for generating aseries of successive scan lines; means for generating apulse-width-modulated signal from a digital input signal input to saidapparatus; and means for applying said pulse-width-modulated signal tosaid print engine to cause said print engine to generate each said scanline as a succession of line-segments, the lengths of which arecontrolled in accordance with said pulse-width-modulated signal toproduce a variable density line screen from said line segments, saidline screen comprising a plurality of columns of said line segments,wherein said digital input signal ranges between maximum and minimumvalues and wherein said pulse-width-modulated signal generating meansgenerates a pulse-width-modulated signal having a predetermined pulsewidth when said digital input signal has the minimum value.
 3. An imageprocessing apparatus according to claim 2, wherein the axes of saidcolumns comprising said line screen are substantially perpendicular tosaid scan lines.
 4. An image processing apparatus according to claim 2,wherein the axes of said columns comprising said line screen extend atan oblique angle to said scan lines.
 5. Image processing apparatusaccording to claim 2, wherein said reference pattern signal generatingmeans includes means for adjusting at least one of the amplitude andoffset of said pattern signal.
 6. An image processing apparatusaccording to claim 2, wherein said generating means includes convertingmeans for converting said digital input signal to an analog videosignal, reference pattern signal generating means for generating ananalog reference pattern signal of predetermined period, and comparingmeans for comparing said converted analog video signal with said analogreference pattern signal and for generating said pulse-width-modulatedsignal on the basis of said comparison.
 7. Image processing apparatusaccording to claim 6, wherein said reference pattern signal generatingmeans generates as said pattern signal a triangular wave signal ofpredetermined period.
 8. An image processing apparatus responsive to adigital input signal, said apparatus comprising: a raster scanning printengine for generating a series of successive scan lines; means forgenerating a pulse-width modulated signal from a digital input signalinput to said apparatus; and means for applying saidpulse-width-modulated signal to said print engine to cause said printengine to generate each said scan line as a succession of line-segments,the lengths of which are controlled in accordance with saidpulse-width-modulated signal to produce a variable density line screenfrom said line segments, said line screen comprising a plurality ofcolumns of said line segments wherein said digital input signal rangesbetween maximum and minimum values and wherein saidpulse-width-modulated signal generating means generates apulse-width-modulated signal having a predetermined pulse width whensaid digital input signal has the maximum value.
 9. Image processingapparatus according to claim 8, wherein the axes of said columnscomprising said line screen are substantially perpendicular to said scanlines.
 10. Image processing apparatus according to claim 8, wherein theaxes of said columns comprising said line screen extend at an obliqueangle to said scan lines.
 11. Image processing apparatus according toclaim 8, wherein said generating means includes converting means forconverting said digital input signal to an analog video signal,reference pattern signal generating means for generating an analogreference pattern signal of predetermined period, and comparing meansfor comparing said converted analog video signal with said analogreference pattern signal and for generating said pulse-width-modulatedsignal on the basis of said comparison.
 12. Image processing apparatusaccording to claim 11, wherein said reference pattern signal generatingmeans includes means for adjusting at least one of the amplitude andoffset of said pattern signal.
 13. Image processing apparatus accordingto claim 11, wherein said reference pattern signal generating meansgenerates as said pattern signal a triangular wave signal ofpredetermined period.
 14. An image processing apparatus for forming animage on a recording medium, said apparatus comprising: video signaloutput means for generating an analog video signal; pattern signalgenerating means for producing a pattern signal of predetermined period;a pulse-width-modulated signal generating means for generating apulse-width modulated signal in accordance with the analog video signalgenerated by said video signal output means and said pattern signalgenerated by said pattern signal generating means; and image formingmeans for scanning lines on a recording medium with a beam in accordancewith said pulse-width-modulated signal generated by said pulse-widthmodulated signal generating means thereby forming an image on saidrecording medium; said image forming means including means forgenerating a synchronizing signal for each line scanned on the recordingmedium, said pattern signal generating means generating the patternsignal of predetermined period in accordance with said synchronizingsignal.
 15. An image processing apparatus according to claim 14, whereinsaid pattern signal generating means generates as said pattern signal atriangular wave signal of predetermined period.
 16. An image processingapparatus according to claim 14, further comprising reference clockgenerating means for generating a reference clock, said pattern signalgenerating means producing a clock for generating said pattern signal bydividing the frequency of said reference clock in accordance with saidsynchronizing signal.
 17. Image processing apparatus according to claim14, wherein said synchronizing signal generating means includesdetecting means for detecting a scanning position of the beam andgenerates the synchronizing signal on the basis of a detection outputfrom said detecting means.
 18. Image processing apparatus according toclaim 14, wherein said pattern signal generating means includes meansfor freely varying a period of the pattern signal generated.
 19. Imageprocessing means according to claim 14, wherein saidpulse-width-modulated signal generating means includes means forcomparing said analog video signal with said pattern signal and forgenerating said pulse-width modulated signal on the basis of thecomparison result.
 20. Image processing apparatus according to claim 14,wherein said pattern signal generating means includes timing changingmeans for changing a timing for generation of the pattern signal inassociation with the scanning line.
 21. Image processing apparatusaccording to claim 14, wherein one period of said pattern signalcorresponds to a plurality of pixels of the analog video signal.
 22. Animage processing apparatus according to claim 14, wherein said patternsignal generating means includes means for adjusting at least one of theamplitude and offset of said pattern signal.
 23. An image processingapparatus according to claim 14, wherein said pattern signal generatingmeans generates as said pattern signal a triangular wave signal ofpredetermined period.
 24. Image processing apparatus according to claim14, further comprising digital video signal generating means forgenerating a digital video signal, wherein said video signal outputmeans includes digital-to-analog converting means for converting thedigital video signal generated by said digital video signal generatingmeans into the analog video signal.
 25. Image processing apparatusaccording to claim 24, wherein said digital video signal ranges betweenmaximum and minimum values and wherein said pulse-width-modulated signalgenerating means generates a pulse-width-modulated signal having apredetermined pulse width when said digital video signal has the minimumvalue.
 26. Image processing apparatus according to claim 24, whereinsaid digital video signal ranges between maximum and minimum values andwherein said pulse-width-modulated signal generating means generates apulse-width-modulated signal having a predetermined pulse width whensaid digital video signal has the maximum value.
 27. Image processingapparatus according to claim 24, wherein said apparatus furthercomprises reference clock generating means for generating a referenceclock, said pattern signal generating means producing a firSt clock forgenerating said pattern signal by dividing the frequency of saidreference clock, and wherein said apparatus further comprises means forgenerating a second clock by dividing the frequency of the referenceclock, said digital video signal generating means generating the digitalvideo signal in synchronism with the second clock.
 28. Image processingapparatus according to claim 27, wherein each of said first and secondclocks is generated in accordance with the synchronizing signal. 29.Image processing apparatus according to claim 24, wherein said digitalvideo signal generating means includes digital video signal output meansfor outputting a digital video signal having a characteristic, andcharacteristic converting means for converting the characteristic ofsaid digital video signal output from said digital video signal outputmeans and for producing a converted digital video signal therefrom andwherein said digital-to-analog converting means converts the converteddigital video signal generated by said characteristic converting meansinto the analog video signal.
 30. Image processing apparatus accordingto claim 29, wherein said characteristic converting means includes meansfor changing a factor for converting the characteristic of said digitalvideo signal for each line scanned by said image forming means. 31.Image processing apparatus according to claim 30, wherein said factorchanging means changes the factor in accordance with the synchronizingsignal.
 32. An image processing apparatus comprising: digital videosignal generating means for generating a digital video signal having acharacteristic; characteristic converting means for converting thecharacteristic of said digital video signal generated by said digitalvideo signal generating means and for producing a converted digitalvideo signal therefrom; digital-to-analog converting means forconverting the converted digital video signal generated by saidcharacteristic converting means into an analog video signal; patternsignal generating means for generating a pattern signal of predeterminedperiod; and pulse-width-modulated signal generating means for generatinga pulse-width-modulated signal in accordance with said analog videosignal and said pattern signal.
 33. An image processing apparatusaccording to claim 32, wherein said digital video signal ranges betweenmaximum and minimum values, and wherein said pulse-width-modulatedsignal generating means generates a pulse-width-modulated signal havingpredetermined pulse width when said digital video signal has the minimumvalue.
 34. An image processing apparatus according to claim 32, whereinsaid digital video signal ranges between maximum and minimum values andwherein said pulse-width-modulated signal generating means generates apulse-width-modulated signal having predetermined pulse width when saiddigital video signal has the maximum value.
 35. An image processingapparatus according to claim 32, wherein said pattern signal generatingmeans includes means for adjusting at least one of the amplitude andoffset of said pattern signal.
 36. Image processing apparatus accordingto claim 32, wherein said pattern signal generating means includes meansfor freely varying a period of the pattern signal generated.
 37. Imageprocessing apparatus according to claim 32, wherein saidpulse-width-modulated signal generating means includes means forcomparing said analog video signal with said pattern signal and forgenerating said pulse-width modulated signal on the basis of thecomparison result.
 38. Image processing apparatus according to claim 32,wherein one period of said pattern signal corresponds to a plurality ofpixels of the digital video signal.
 39. An image processing apparatusaccording to claim 32, wherein said characteristic converting meanscomprises storage means containing digital information for providing atleast one non-linear transformation of said digital video signal.
 40. Animage processing apparatus according to claim 39, wherein said storagemeans comprises a read only memory for storing a digital look-up tablefor gamma correction.
 41. An image processing apparatus according toclaim 32, further comprising image forming means for scanning successivelines on a recording medium with a beam in accordance with saidpulse-width-modulated signal generated by said pulse-width-modulatedsignal generating means thereby forming an image on said recordingmedium, and wherein said characteristic converting means includes meansfor changing the factor converting the characteristic of said digitalvideo signal for each of the successive lines scanned by said imageforming means.
 42. Image processing apparatus according to claim 41,wherein said pattern signal generating means includes timing changingmeans for changing a timing for generation of the pattern signal inassociation with the scanning line.
 43. Image processing apparatusaccording to claim 41, wherein said image forming means includes meansfor generating a synchronizing signal for each line scanned on therecording medium, said pattern signal generating means generating thepattern signal of predetermined period in accordance with saidsynchronizing signal.
 44. Image processing apparatus according to claim37, wherein said synchronizing signal generating means includesdetecting means for detecting a scanning position of the beam andgenerates the synchronizing signal on the basis of a detection outputfrom said detecting means.
 45. Image processing apparatus according toclaim 43, wherein said factor changing means changes the factor inaccordance with the synchronizing signal.
 46. Image processing apparatusaccording to claim 43, further comprising reference clock generatingmeans for generating a reference clock, said pattern signal generatingmeans producing a clock for generating said pattern signal by dividingthe frequency of said reference clock in accordance with saidsynchronizing signal.
 47. An apparatus for generating an image from adigital video signal comprising: A. a raster scanning print engine forgenerating a series of successive scanning lines; B. a digital look uptable addressable by said digital video signal; C. means for generatingfrom said digital video signal a pulse-width-modulated signal, saidpulse-width-modulated signal generating means comprising: (1) means forapplying said digital video signal as an address to said digital look uptable for producing in accordance with said digital look up table aresultant gamma corrected digital signal; (2) means for converting saidresultant gamma corrected digital signal to an analog video signal; (3)means for generating a periodic analog reference signal of predeterminedperiod; and (4) means for comparing said analog video signal to saidperiodic analog reference signal thereby to produce saidpulse-width-modulated signal; and D. means for applying said pulse-widthmodulated signal to said print engine for generating each said scanningline as a succession of line-segments, the length of which arecontrolled in accordance with said pulse-width-modulated signal toproduce a variable density line screen from said line segments, an axisof symmetry of the line screen being substantially normal to thescanning lines.
 48. An apparatus for generating an image from a digitalvideo signal according to claim 47, wherein said periodic analogreference signal generating means generates as said analog referencesignal a triangular wave signal of a predetermined period.
 49. Anapparatus for generating an image according to claim 47, wherein saiddigital video signal ranges between maximum and minimum values andwherein said pulse-width-modulated signal generating means generates apulse-width-modulated signal having a predetermined pulse width whensaid digital video signal has the minimum value.
 50. An apparatus forgenerating an image according to claim 47, wherein said digital videosignal ranges between maximum and minimum values and wherein saidpulse-width-modulated signal generating means generates apulse-width-modulated signal having predetermined pulse width when saiddigital video signal has the maximum value.
 51. An apparatus forgenerating an image according to claim 47, wherein periodic analogreference signal generating means includes means for adjusting at leastone of the amplitude and offset of said analog reference signal. 52.Apparatus according to claim 47, further comprising table changing meansfor changing the digital look up table utilized in association with thescanning line generated by said raster scanning print engine. 53.Apparatus according to claim 47, wherein one period of the analogreference signal corresponds to a plurality of pixels of the digitalvideo signal.
 54. Apparatus according to claim 19, wherein said rasterscanning print engine includes image forming means for scanning lines ona recording medium with a beam in accordance with saidpulse-width-modulated signal generated by said pulse-width-modulatedsignal generating means thereby forming an image on said recordingmedium, said image forming means including means for generatinga.synchronizing signal for each line scanned on the recording medium,said periodic analog reference signal generating means generating theperiodic analog reference signal of predetermined period in accordancewith said synchronizing signal.
 55. Image processing apparatus accordingto claim 54, wherein said synchronizing signal generating means includesdetecting means for detecting a scanning position of the beam andgenerates the synchronizing signal on the basis of a detection outputfrom said detecting means.
 56. An image processing apparatus responsiveto a digital video signal input thereto, said apparatus comprising: adigital-to-analog converting means for converting the digital videosignal input to said apparatus into an analog video signal; a patternsignal generating means for generating a pattern signal of apredetermined period; and a pulse-width-modulated signal generatingmeans for generating a pulse-width-modulated signal in accordance withsaid converted analog video signal and said pattern signal.
 57. An imageprocessing apparatus according to claim 56, wherein said pattern signalgenerating means generates as said pattern signal a triangular wavesignal of predetermined period.
 58. Image processing apparatus accordingto claim 56, wherein said digital video signal ranges between maximumand minimum values and wherein said pulse-width-modulated signalgenerating means generates a pulse-width-modulated signal having apredetermined pulse width when said digital video signal has the minimumvalue.
 59. Image processing apparatus according to claim 56, whereinsaid digital video signal ranges between maximum and minimum values andwherein said pulse-width-modulated signal generating means generates apulse-width-modulated signal having a predetermined pulse width whensaid digital video signal has the maximum value.
 60. Image processingapparatus according to claim 56, wherein said pulse-width-modulatedsignal generating means includes means for comparing said analog videosignal with said pattern signal and for generating said pulse-widthmodulated signal on the basis of the comparison result.
 61. Imageprocessing apparatus according to claim 56, wherein one period of saidpattern corresponds to a plurality of pixels of the digital videosignal.
 62. Image processing apparatus according to claim 56, furthercomprising image forming means for forming an image by lines on arecording medium in accordance with said pulse-width-modulated signalgenerated by said pulse-width modulated signal generating means, saidimage forming means including means for generating a synchronizingsignal for each line on the recording medium, said pattern signalgenerating means generating the pattern signal of predetermined periodin accordance with said synchronizing signal.
 63. Image processingapparatus according to claim 62, wherein said image forming means scanslines on the recording medium with a beam in accordance with saidpulse-width-modulated signal, thereby forming the image on the recordingmedium, and wherein said synchronizing signal generating means includesdetecting means for detecting a scanning position of the beam, andgenerates the synchronizing signal on the basis of a detection outputfrom said detecting means.
 64. Image processing apparatus according toclaim 62, further comprising reference clock generating means forgenerating a reference clock, said pattern signal generating meansproducing a clock for generating said pattern signal by dividing thefrequency of said reference clock in accordance with said synchronizingsignal.
 65. Image processing apparatus according to claim 62, whereinsaid pattern signal generating means includes timing changing means forchanging a timing for generation of the pattern signal in associationwith each line.
 66. Image processing apparatus according to claim 56,further comprising digital video signal input means for inputting thedigital video signal having a characteristic and characteristicconverting means for converting the characteristic of said digital videosignal input by said digital video signal input means and for producinga converted digital video signal therefrom, wherein saiddigital-to-analog converting means converts the converted digital videosignal generated by said characteristic converting means into the analogvideo signal.
 67. Image processing apparatus according to claim 66,wherein said characteristic converting means includes a table forentering as an address the digital video signal input from said digitalvideo signal input means, the converted digital video signal beingproduced from said table.
 68. Image processing apparatus according toclaim 67, wherein a plurality of tables for entering as an address thedigital video signal input from said digital video signal input meansare provided, and further comprising image forming means for scanninglines on a recording medium with a beam in accordance with saidpulse-width-modulated signal generated by said pulse-width-modulatedsignal generating means thereby forming an image on said recordingmedium, and table changing means for changing the table utilized inassociation with the scanning line.
 69. Image processing apparatusaccording to claim 68, wherein said image forming means includes meansfor generating a synchronizing signal for each line scanned on therecording medium, and said table changing means changes the table inaccordance with the synchronizing signal.
 70. Image processing apparatusaccording to claim 69, wherein said synchronizing signal generatingmeans includes detecting means for detecting a scanning position of thebeam and generates the synchronizing signal on the basis of a detectionoutput from said detecting means.
 71. Image processing apparatuscomprising: digital video signal generating means for generating adigital video signal having a characteristic; characteristic convertingmeans for converting the characteristic of said digital video signalgenerated by said digital video signal generating means and forproducing a converted digital video signal therefrom, saidcharacteristic converting means including changing means forperiodically changing a plurality of factors for converting thecharacteristic of said digital video signal at a predetermined intervalthereby to convert the characteristic of said digital video signalgenerated by said digital video signal generating means and to producesaid converted digital video signal by using said plurality of factors;and image forming means for forming an image in accordance with saidconverted digital video signal, said image forming means forming saidimage by scanning lines on a recording medium in accordance with saidconverted digital video signal; wherein said characteristic convertingmeans uses the same ones of said plurality of factors for at least onescanning line scanned by said image forming means, and said changingmeans changes said plurality of factors after scanning of said at leastone scanning line scanned by said scanning means.
 72. Image processingapparatus according to claim 71, wherein said image forming meansincludes pulse-width-modulated signal generating means for generating apulse-width-modulated signal in accordance with the converted videosignal.
 73. Image processing apparatus according to claim 71, whereinsaid characteristic converting means comprises storage means containingdigital information for providing at least one non-linear transformationof said digital video signal.
 74. Image processing apparatus accordingto claim 73, wherein said storage means comprises a read only memory forstoring a digital look-up table for gamma correction.
 75. Imageprocessing apparatus according to claim 71, wherein said characteristicconverting means includes a table for entering as an address the digitalvideo signal generated from said video signal generating means, theconverted digital video signal being produced from the table.
 76. Imageprocessing apparatus according to claim 75, wherein a plurality oftables are provided for entering as an address the digital video signalgenerated from the video signal generating means, and wherein saidchanging means changes the table for each line scanned by said imageforming means.
 77. Image processing apparatus according to claim 76,wherein said image forming means includes digital-to-analog convertingmeans for converting the converted digital video signal generated bysaid characteristic converting means into an analog video signal,pattern signal generating means for generating a pattern signal ofpredetermined period, pulse-width-modulated signal generating means forgenerating a pulse-width-modulated signal in accordance with said analogvideo signal and said pattern signal, and a raster scanning print enginefor generating a series of successive scan lines with a beam inaccordance with said pulse-width-modulated signal.
 78. Image processingapparatus according to claim 77, wherein said digital video signalranges between maximum and minimum values, and wherein saidpulse-width-modulated signal generating means generates apulse-width-modulated signal having predetermined pulse width when saiddigital video signal has the minimum value.
 79. Image processingapparatus according to claim 77, wherein said digital video signalranges between maximum and minimum values and wherein saidpulse-width-modulated signal generating means generates apulse-width-modulated signal having predetermined pulse width when saiddigital video signal has the maximum value.
 80. Image processingapparatus according to claim 77, wherein said pattern signal generatingmeans includes an integrator for entering a clock having a predeterminedperiod and for generating a triangular wave signal as the pattern signalon the basis of the clock having a predetermined period.
 81. Imageprocessing apparatus according to claim 77, wherein said image formingmeans includes means for generating a synchronizing signal for each linescanned on the recording medium, and said changing means changes thetable in accordance with the synchronizing signal.
 82. Image processingapparatus according to claim 81, wherein said synchronizing signalgenerating means includes detecting means for detecting a scanningposition of the beam and generates the synchronizing signal on the basisof a detection output from said detecting means.
 83. Image processingapparatus according to claim 81, wherein said pattern signal generatingmeans generates the pattern signal of predetermined period in accordancewith said synchronizing signal.
 84. Image processing apparatus accordingto claim 81, wherein one period of said pattern corresponds to aplurality of pixels of the digital video signal.
 85. Image processingapparatus for forming an image, said apparatus comprising: video signaloutput means for generating an analog video signal; pattern signalgenerating means for producing a pattern signal of predetermined period,one period of said pattern signal corresponding to a plurality of pixelsof the analog video signal; a pulse-width-modulated signal generatingmeans for generating a pulse-width-modulated signal in accordance withthe analog video signal generated by said video signal output means andsaid pattern signal generated by said pattern signal generating means;and image forming means for forming an image in accordance with saidpulse-width-modulated signal generated by said pulse-width-modulatedsignal generating means.
 86. Image processing apparatus according toclaim 85, wherein said pattern signal generating means generates as saidpattern signal a triangular wave signal of predetermined period. 87.Image processing apparatus according to claim 85, wherein said patternsignal generating means includes means for adjusting at least one of theamplitude and offset of said pattern signal.
 88. Image processingapparatus according to claim 85, wherein said image forming means scanslines on the recording medium with a beam in accordance with saidpulse-width-modulated signal generated by said pulse-width-modulatedsignal generating means thereby forming an image on said recordingmedium, said image forming means including means for generating asynchronizing signal for each line scanned on the recording medium, saidpattern signal generating means generating the pattern signal ofpredetermined period in accordance with said synchronizing signal. 89.Image processing apparatus according to claim 88, wherein saidsynchronizing signal generating means includes detecting means fordetecting a scanning position of the beam and generates thesynchronizing signal on the bases of a detection output from saiddetecting means.
 90. Image processing apparatus according to claim 88,further comprising reference clock generating means for generating areference clock, said pattern signal generating means producing a clockfor generating said pattern signal by dividing the frequency of saidreference clock in accordance with said synchronizing signal.
 91. Imageprocessing apparatus according to claim 85, further comprising digitalvideo signal generating means for generating a digital video signal,wherein said video signal output means includes digital-to-analogconverting means for converting the digital video signal generated bysaid digital video signal generating means into the analog video signal.92. Image processing apparatus according to claim 91, wherein saiddigital video signal generating means includes digital video signaloutput means for outputting a digital video signal having acharacteristic, and characteristic converting means for converting thecharacteristic of said digital video signal output from said digitalvideo signal output means and for producing a converted digital videosignal therefrom and wherein said digital-to-analog converting meansconverts the converted digital video signal generated by saidcharacteristic converting means into the analog video signal.
 93. Imageprocessing apparatus according to claim 91, wherein said digital videosignal ranges between maximum and minimum values and wherein saidpulse-width-modulated signal generating means generates a pulse-widthmodulated signal having a predetermined pulse width when said digitalvideo signal has the minimum value.
 94. Image processing apparatusaccording to claim 91, wherein said digital video signal ranges betweenmaximum and minimum values and wherein said pulse-width-modulated signalgenerating means generates a pulse-width-modulated signal having apredetermined pulse width when said digital video signal has the maximumvalue.
 95. Image processing apparatus according to claim 91, wherein oneperiod of the pattern signal corresponds to a plurality of pixels of thedigital video signal.
 96. Image processing apparatus according to claim91, wherein said apparatus further comprises reference clock generatingmeans for generating a reference clock, said pattern signal generatingmeans producing a fIrst clock for generating said pattern signal bydividing the frequency of said reference clock and wherein saidapparatus further comprises means for generating a second clock bydividing the frequency of the reference clock, said digital video signalgenerating means generating the digital video signal in synchronism withthe second clock.
 97. Image processing apparatus according to claim 85,wherein one period of said pattern signal corresponds to two pixels ofthe analog video signal.
 98. Image processing apparatus according toclaim 85, wherein one period of said pattern signal corresponds to threepixels of the analog video signal.
 99. An image processing apparatusresponsive to a digital input signal, said apparatus comprising: rasterscanning image forming means for generating a series of successive scanlines; means for digitally converting a characteristic of a digitalinput signal input to said apparatus to produce a converted digitalsignal; means for generating a pulse-width-modulated signal from saidconverted digital signal; and means for applying saidpulse-width-modulated signal to said image forming means to cause saidimage forming means to generate each said scan line as a succession ofline-segments, the lengths of which are controlled in accordance withsaid pulse-width-modulated signal to produce a variable density linescreen from said line segments, said line screen comprising a pluralityof columns of said line segments.
 100. An image processing apparatusaccording to claim 99, wherein said characteristic converting meansincludes means for gamma-correcting said digital input signal.
 101. Animage processing apparatus responsive to a digital input signal, saidapparatus comprising: raster scanning image forming means for generatinga series of successive scan lines; means for digitally nonlinearlygenerating a generated digital signal from a digital input signal inputto said apparatus; means for generating a pulse-width-modulated signalfrom said generated digital signal; and means for applying saidpulse-width-modulated signal to said image forming means to cause saidimage forming means to generate each said scan line as a succession ofline-segments, the lengths of which are controlled in accordance withsaid pulse-width-modulated signal to produce a variable density linescreen from said line segments, said line screen comprising a pluralityof columns of said line segments.
 102. An image processing apparatusaccording to claim 101, wherein said nonlinear generating means includesmeans for gamma-correcting said input signal.
 103. In a pixel recordingpulse signal generation method in which tone data signals representingrespective pixels of an image are transformed into respective pixelrecording pulse signals each having a time width proportional to thevalue of the respective tone data signal, and in which said pixelrecording pulse signals are used to print respective dots in respectivepixels along scanning lines to reproduce said image, wherein only onedot is printed in each pixel of the reproduced image, the improvementwherein the pixels along each scanning line of the reproduced image aredivided into pairs of adjacent pixels, each pair consisting of a firstpixel and a second pixel which succeeds the first pixel, and wherein theend of the pixel recording pulse corresponding to the first pixel ismade to coincide with the beginning of the pixel recording pulse signalcorresponding to the second pixel, thus causing the dots printed in thefirst and second pixels to be contiguous over the boundary in the firstand second pixels.
 104. A scanning recording type printing devicecomprising: memory means for storing tone data signals for a scanningline; means for generating a triangular comparison data signal for eachof a plurality of pairs of pixels of an image to be recorded; means forcomparing said stored tone data signals with said comparison data signaland for generating a pixel recording pulse based on the comparison; asemiconductor laser circuit for producing laser light based on saidpixel recording pulse signal; means for recording an image correspondingto said pixel recording pulse signal on a recording medium by sweepingsaid laser light across said recording medium in a scanning direction torecord a dot in each of said plurality of pixels; and timing treatmentmeans for controlling said memory means, said comparison data signalproduction means, said pixel recording pulse signal production means andsaid image recording means such that no interruptions in the scanningdirection exist between dots in first ones of said plurality of pixelsand dots in respective succeeding ones of said plurality of pixelsthereby producing dots which are continuous in the scanning directionacross the boundaries of the said first and succeeding pixels, whereineach of said plurality of dots has a width in a direction perpendicularto the scanning direction that is constant and a length in the scanningdirection that is proportional to a respective one of said tone datasignals.
 105. A scanning recording type printing device comprising:memory means for storing tone data signals for a scanning line; meansfor generating a triangular comparison data signal for each of aplurality of pairs of pixels of an image to be recorded; means forcomparing said stored tone data signals with said comparison data signaland for generating a pixel recording pulse based on the comparison; asemiconductor laser circuit for producing laser light based on saidpixel recording pulse signal; means for recording an image correspondingto said pixel recording pulse signal on a recording medium by sweepingsaid laser light across said recording medium in a scanning direction torecord a dot in each of said plurality of pixels; and timing treatmentmeans for controlling said comparison data signal production means, saidpixel recording pulse signal production means and said image recordingmeans such that no interruptions in the scanning direction exist betweendots in first ones of said plurality of pixels and dots in respectivesucceeding ones of said plurality of pixels thereby producing dots whichare continuous in the scanning direction across the boundaries of thesaid first and succeeding pixels, wherein each of said plurality of dotshas a width in a direction perpendicular to the scanning direction thatis constant and a length in the scanning direction that is proportionalto a respective one of said tone data signals.
 106. A scanning recordingtype printing method in which an image is printed by printing a dot ineach of a plurality of pixels of the printed image, wherein the lengthof each dot in a scanning direction is proportional to respective tonedata sampled from an original image and the widths of each dot in adirection perpendicular to the scanning direction is constant, saidmethod comprising the step of: locating dots within selected pixels in ascanning line of the printed image such that the dots in the selectedpixels are printed at second ends of the selected pixels, said secondends of said selected pixels being adjacent to first ends of respectivesucceeding pixels, wherein no interruptions in the scanning directionexist between the dots in the selected pixels and dots in the respectivesucceeding pixels.
 107. A scanning type printing method according toclaim 106, wherein said selected pixels in said scanning line compriseodd-numbered pixels in said scanning line and the succeeding pixelcomprises even-numbered pixels in said scanning line.
 108. A scanningtype printing method according to claim 106, wherein said selectedpixels in said scanning line comprise even-numbered pixels in saidscanning line and the succeeding pixels comprise odd-numbered pixels insaid scanning line.
 109. A scanning recording type printing devicecomprising: memory means for storing tone data signals for a scanningline; means for generating a triangular comparison data signal for eachof a plurality of pairs of pixels of an image to be recorded; means forcomparing said stored tone data signals with said comparison data signaland for generating a pixel recording pulse based on the comparison; asemiconductor laser circuit for producing laser light based on saidpixel recording pulse signal; means for recording an image correspondingto said pixel recording pulse signal on a recording medium by sweepingsaid laser light across said recording medium in a scanning direction torecord a tone dot in each of said plurality of pixels; and timingtreatment means for controlling said memory means, said comparison datasignal production means, said pixel recording pulse signal productionmeans and said image recording means such that tone dots within selectedpixels in a scanning line of the recorded image are recorded at secondends of said selected pixels, said second ends of said selected pixelsbeing adjacent to first ends of respective succeeding pixels, whereinsaid tone dots have a constant width in a direction perpendicular to thescanning direction that is constant and each tone dot has a length inthe scanning direction that is proportional to a respective one of saidtone data signals.
 110. A scanning recording type printing deviceaccording to claim 109, wherein said selected pixels in said scanningline comprise odd-numbered pixels in said scanning line and thesucceeding pixel comprise even-numbered pixels in said scanning line.111. A scanning recording type printing device according to claim 109,wherein said selected pixels in said scanning line compriseseven-numbered pixels in said scanning line and the succeeding pixelscomprise odd-numbered pixels in said scanning line.
 112. A scanningrecording type printing device according to claim 109, furthercomprising means for controlling a screen angle at which the tone dotsare recorded by controlling said comparison data signal productionmeans.
 113. A scanning recording type printing device comprising: memorymeans for storing tone data signals for a scanning line; means forgenerating a triangular comparison data signal for each of a pluralityof pairs of pixels of an image to be recorded; means for comparing saidstored tone data signals with said comparison data signal and forgenerating a pixel recording pulse based on the comparison; asemiconductor laser circuit for producing laser light based on saidpixel recording pulse signal; means for recording an image correspondingto said pixel recording pulse signal on a recording medium by sweepingsaid laser light across said recording medium in a scanning direction torecord a tone dot in each of said plurality of pixels; and timingtreatment means for controlling said comparison data signal productionmeans, said pixel recording pulse signal production means and said imagerecording means such that tone dots within selected pixels in a scanningline of the recorded image are recorded at second ends of said selectedpixels, said second ends of said selected pixels being adjacent to firstends of respective succeeding pixels, wherein said tone dots have aconstant width in a direction perpendicular to the scanning directionthat is constant and each tone dot has a length in the scanningdirection that is proportional to a respective one of said tone datasignals.
 114. A scanning recording type printing device according toclaim 113, wherein said selected pixels in said scanning line compriseodd-numbered pixels in said scanning line and the succeeding pixelcomprise even-numbered pixels in said scanning line.
 115. A scanningrecording type printing device according to claim 113, wherein saidselected pixels in said scanning line comprise even-numbered pixels insaid scanning line and the succeeding pixels comprise odd-numberedpixels in said scanning line.
 116. A scanning recording type printingdevice according to claim 113, further comprising means for controllinga screen angle at which the tone dots are recorded by controlling saidcomparison data signal production means.
 117. A scanning recording typeprinting method in which an image is printed by printing a dot in eachof a plurality of pixels of the printed image, wherein the length ofeach dot in a scanning direction is proportional to respective tone datasampled from an original image and the width of each dot in a directionperpendicular to the scanning direction is constant, said methodcomprising the steps of: locating second ends of first pixels and secondends of dots in said first pixels in coincidence with each other, andlocating first ends of respective second pixels succeeding said firstpixels in a scanning line and first ends of dots in said second pixelsin coincidence with each other such that no interruptions in thescanning direction exist between said dots in said first pixels and saiddots in said second pixels, thereby causing the printed image to haveone continuous dot per pair of pixels; or locating first ends of firstpixels and first ends of dots in said first pixels in coincidence witheach other and locating second ends of respective second pixelssucceeding said first pixels in a scanning line and second ends of dotsin said second pixels in coincidence with each other such that nointerruptions in the scanning direction exist between white areaswithout said dots recorded within said first pixels and white areaswithout said dots recorded within said second pixels, thereby causingthe printed image to have one continuous white area per pair of pixels.118. A method according to claim 103, wherein said pixel recording pulsesignals are formed based on the tone data signals and a comparison datasignal.
 119. A method according to claim 118, wherein said comparisondata signal is a triangular comparison signal for each of a plurality ofpairs of pixels of an image to be recorded.
 120. A method according toclaim 103, wherein the tone data signals are obtained bycharacteristic-conversion of an input digital video signal.
 121. Amethod according to claim 118, wherein said pixel recording signals areobtained using a signal obtained by analog-conversion of the tone datasignals.
 122. An image processing apparatus operable in conjunction witha photosensitive member, said apparatus comprising: digital video signalgenerating means for generating a digital video signal having acharacteristic; characteristic conversion means for converting thecharacteristic of the digital video signal generated by said digitalvideo signal generating means to generate a characteristic-converteddigital video signal; and electro-photographic system image formingmeans for forming an image by line-scanning said photosensitive memberwith an optical beam in accordance with the characteristic-converteddigital video signal, said characteristic conversion means comprisingchanging means for periodically changing a plurality of gamma-conversioncharacteristics for converting the digital video signal, duringformation of an image, and said plurality of characteristics are set sothat a change in characteristics among the plurality of gamma-conversioncharacteristics in a light area of an image may be greater than that ina dark area of an image.
 123. An apparatus according to claim 122,wherein said changing means is arranged so as to change saidgamma-conversion characteristics according to line-scanning by saidimage forming means.
 124. An image processing apparatus, comprising:video signal input means for inputting a digital video signal capable ofrepresenting a gradation level with a plurality of bits for each pixelof an image; pattern signal generating means for generating a pluralityof different pattern signals, each of which may have one of a pluralityof different periods; selection means for selecting one of the pluralityof periods of the pattern signal generated by the pattern signalgenerating means; pulse-width modulated signal generating means forgenerating a pulse-width modulated signal having a pulse widthcorresponding to a gradation level of the digital video signal input bysaid digital video signal input means based on the digital video signaland a pattern signal generated by said pattern signal generating means;and image forming means for forming an image in accordance with thepulse-width modulated signal generated by said pulse-width modulatedsignal generating means.
 125. An apparatus according to claim 124,wherein at least one of the periods of said pattern signal generated bysaid pattern signal generating means corresponds to a plurality ofpixels of said digital video signal.